Gasifier system and method

ABSTRACT

A solid fuel fed combustor method having a first chamber portion with an inlet feeding a metered amount of a solid fuel to a first burner stage having a first traveling conveyor firebelt. A metered amount of air is introduced in progressively increasing proportions along the length thereof to cause endothermic reduction of the solid fuel. The first stage feeds a second burner stage having a second traveling conveyor firebelt. Air is introduced in a progressively decreasing amount along the length of the second traveling conveyor firebelt to induce exothermic combustion and decomposition of fuel. The amount of air introduced and speed of the conveyors are controlled to minimize the quantity of carbon monoxide and nitrogen oxides and other pollutants. Radiative energy generated from fuel traveling on the traveling conveyor firebelts is reflected on fuel traveling on the first firebelt.

REFERENCE TO RELATED APPLICATIONS

This is division of application Ser. No. 10/061,362, filed Feb. 4, 2002now U.S. Pat. No. 6,532,879 which in turn is a continuation of Ser. No.09/138,020, filed Aug. 21, 1998 now abandoned.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION

This invention relates to gasifier systems and methods for the efficientconversion of solid fuels to usable heat energy.

DESCRIPTION OF THE PRIOR ART

The prior art is best exemplified by U.S. Pat. No. 5,284,103 entitledBIO-MASS BURNER CONSTRUCTION by Hand et al, which is a division of U.S.Pat. No. 5,178,076 entitled BIO-MASS BURNER CONSTRUCTION issued Jan. 12,1993 to the same inventors. In these patents, the burner utilizes afirst burning chamber having a falling fuel entrained bed zonepositioned above a traveling grate having a porous metallic woven belt.Primary air is directed through the porous belt to establish anoxygen-starved first burning chamber. A second burning chamber in fluidcommunication with the first burning chamber has a restricted diameterand effectively provides a hot-air gas nozzle. In larger sized units, aplurality of conveyors constitutes the traveling grate with theconveyors being arranged in head-to-head stepped relationship so thatunburned fuel received by gravity from the entrained bed zone isagitated or jostled to enhance its burning.

SUMMARY OF THE INVENTION

According to the present invention, firebelts ensure that the heat lossfrom heating unnecessary air is minimized. The quantity of air at eachpoint in the combustion process is stringently controlled. This aircontrol benefits the combustion process in three ways:

First, by minimizing the heat loss of the combustion process, thisminimizes the amount of carbon monoxide which is produced. Carbonmonoxide, a priority pollutant, is produced directly proportional to thecombustion temperature. Therefore, by minimizing excess air, thequantity of carbon monoxide produced is minimized.

Secondly, nitrogen oxides, another priority pollutant, is produced bycombining the nitrogen in the air with the oxygen in the air. Thiscombination of nitrogen and oxygen only occurs at high temperatures. Thehigher the temperature, the greater the quantity of nitrogen oxides thatare produced. While the combustor of the present invention utilizes hightemperatures, the formation of nitrogen and oxides is minimized sincethere is no excess oxygen to combine with the nitrogen. All the oxygenis used in the combustion process.

Thirdly, by minimizing the amount of air supplied to the combustionprocess, the amount of energy required to move air to the combustor isminimized. Electrical energy costs are typically 20% less than theelectricity costs used in similar combustion systems where the air isnot stringently controlled.

A further feature of the present invention is in the use of reflectedinfrared energy. Heat is a form of electromagnetic energy similar tolight where the rays can be refracted or reflected. Radiation producedfrom heat is of a longer wavelength than visible light and is calledinfrared radiation. By reflecting a certain amount of the heat producedfrom a combustion process, this invention is able to supply heat to thegasification process. The reflected heat will be of benefit in two ways:

Firstly, the heat is reflected to a point where the heat can be used toassist the combustion process. This is generally where the fuel firstenters the combustion process. At this point, the fuel must be heatedand the water removed. These processes require addition of energy whichcan be added for heat of the combustion of a part of the fuel or fromthe reflected energy. Using a part of the fuel to preheat the remainingfuel is inefficient, leaving less total heat available for production ofelectricity. Using reflected heat (in the form of radiant energy)removes or minimizes this inefficiency. The second way this benefits theoverall combustion process is in that the energy is transferred in abeneficial way—not wasted by irradiating and heating in the combustionchamber. Heat that is absorbed by the combustion chamber is generallywasted since there is no direct benefit from this radiation. A smallportion is used in the maintenance of the necessary combustiontemperature but the majority of irradiating heat is wasted as low-levelheat irradiating from the combustor exterior. Reflective heat added tothe fuel will benefit the overall combustion efficiency, and this is afeature of the present invention.

Another feature of the invention is that the speed of the conveyor driveand the rate of inlet air and the control of inlet air is much moreclosely controlled so as to achieve high efficiency. Still anotherfeature of the invention is that the fuel feed ramping is based onthermal conditions at the boiler output.

Still another feature of the invention is that the induced draft fancontrol is based on the draft pressure and boiler airflow rate.

Still another feature of the invention is in the method to controlcatalyst feed levels based on pollutant levels in the stack as measuredat a continuous emission monitoring system point.

Finally, the invention features a computer control system which is basedon operational parameters sensed at different stages in the process.

Accordingly, the object of the invention is to provide an improvedgasifier system.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the inventionwill become more apparent when considered with the followingspecification and accompanying drawings wherein:

FIG. 1 is a diagrammatic sectional illustration of a gasifier systemincorporating the invention,

FIG. 2 is a cross-sectional illustration of a walking floor trailer andinclined conveyor incorporated in the invention for feeding fuel to thecombustor unit,

FIG. 3 is a diagrammatic illustration of the bin feeder and rotary airlock incorporated in the invention,

FIG. 4 is a diagrammatic illustration of the boiler for convertingthermal energy to steam,

FIG. 5 is a diagrammatic illustration of the cyclone and baghouseparticulate collection division,

FIG. 6 is a diagrammatic illustration of the scrubber for removing acidgases from the exhaust gas stream,

FIGS. 7A, 7B and 7C are flow diagrams illustrating the combustionprocess and location of various sensors and control elementsincorporated in the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the gasifier system comprises an inlet feedconveyor and fuel metering unit 10, a gasifier 11, a gasifier firetubewith connection to boiler 12, the boiler itself 13, a cyclone 14,baghouse 15, scrubber 16, and computer control system 17 which computercontrol system is diagrammatically illustrated in FIGS. 7A, 7B and 7C.

Inlet Feed Conveyor With Fuel Metering Unit 10

This section delivers the prepared fuel to the gasifier. It includes thebin conveyor and rotary airlock. The computer control system determinesthe required fuel flow for proper gasification and the quantity tosustain the output of steam from the boiler. It then determines therequired speed of the conveyor and the airlock.

Walking Floor Trailer and Inclined Conveyor (FIG. 2)

The walking floor trailer 2-10 is used to store and deliver fuel to thecombustor with the inclined conveyor. The bin conveyor 2-12 monitors theamount of fuel and calls for additional fuel from the walking floortrailer when necessary.

The walking floor trailer 2-10 works in the following manner: The floor2-14 is comprised of a number of strips that independently move. To movethe fuel to the rear of the trailer (left end in FIG. 2), the strips allmove together rearward. At the end of the cycle (approximately fourinches), each strip independently moves forward, leaving the fuelundisturbed. This cycle is repeated as required to move the fuelrearward as much as necessary. A monitor or sensor 2-15 is located atthe input end of inclined conveyor 2-11. The inclined conveyor 2-11monitors the quantity of fuel located at the end of the trailer (thebeginning of the inclined conveyor) and moves the fuel rearward in thewalking floor trailer as required to maintain sufficient fuel in theinclined conveyor.

Bin Feeder and Rotary Air Lock

The bin feeder (FIG. 3) receives the fuel from the inclined elevator2-11 (FIG. 2). The bin feeder is used to meter the fuel to thecombustor. Depending upon the quantity of fuel required by the combustorto produce sufficient stream, the bin feeder 3-11 speed will be variedto introduce sufficient fuel to the combustor. A sensor 3-10 is used onthe bin feeder to ensure that sufficient fuel is present on the binfeeder belt. If additional fuel is needed, a signal is sent to theinclined conveyor/walking floor trailer (FIG. 2) for additional fuel.

The rotary air lock 3-12 is used to provide a mechanical seal tominimize the quantity of unwanted air introduced into the combustor.

Gasifier 11

The fuel gasification process takes place in gasifier 11 shown in FIG.1. device. This process is described hereafter in the section entitled“COMBUSTION PROCESS DESCRIPTION”. The area of this gasification isreferred to as stage 1. The gasification in this stage is controlledtotally by the computer control system by taking the followingparameters and the system computer determines the variables that need tochange or remain the same during the gasification process:

Stage 1 a:

-   -   Solid fuel feed rate,    -   Airflow rate,    -   Oxygen concentration,    -   Firebelt 20 speed,    -   Gasification temperature,    -   Draft pressure.

Stage 1 b:

-   -   Air flow rate,    -   Oxygen concentration,    -   Firebelt 21 speed,    -   Oxidation temperature,    -   Draft pressure.        Firebelt (Conveyor) Air Flows

The variable control of air through the firebelts (conveyors) 20, 21 isthe primary reason for the extremely low air emissions from thecombustor disclosed herein. Controlling the amount of air that passesthrough each unit area of the firebelts governs the quantity and qualityof the combustion process.

The combustion process requires two components; fuel and an oxidizer.Normally, air is used as an oxidizer but air contains many gases, mostof which do not contribute to the oxidation process. In fact, the othergases in air can have a deleterious effect on the overall combustionefficiency.

The entire combustion process is a series of discrete steps where heatis either created or it is used. In general, the following steps occurin the solid fuel combustion process:

Firebelt 20 Heat solid fuel to operating temperature requires energyRemove adsorbed water from solid fuel requires energy Heat combustionair to operating temperature requires energy Gasify fuel components insolid fuel requires energy Decomposc fuel gases into elemental fuelsrequires energy Reflected heat from firebelt 21 produces energy Combustsmall percentage of solid fuel for heat produces energy Net energy:Produces very little energy Heat loss: Negligible Firebelt 20 Heatcombustion air to operating temperature requires energy Additionalbeating of fuel and ash from requires energy 1,500° F to 2,500° F.Reflect heat to firebelt 20 requires energy Combust carbon into carbonmonoxide produces energy Preheat air from hot bottom ash produces energyNet energy: Produces little energy Heat loss: Small - from hot bottomash Firetube 12 Heat combustion air to operating temperature requiresenergy Additional heating of fuel and ash from requires energy 2,500° F.to 4,000° F. Combust fuels produces energy Preheat combustion air fromcombustor refractory produces energy Net energy: Produces significantenergy Heat loss: Negligible Boiler Superheater Heat transferred tosteam requires energy Net energy: Requires significant energy Heat loss:Negligible Boiler 11 Heat transferred to boil water requires energy Netenergy: Requires significant energy Heat loss: Negligible BoilerPreheater Heat transferred to water requires energy Net energy: Requiressignificant energy Heat loss: Negligible Cyclone 14 Heat lost viaradiant heating produces energy Heat lost in fly-ash produces energy Netenergy: Produces unusable energy Heat loss: Significant Baghouse 15 Heatlost via radiant heating produces energy Heat lost in fly-ash producesenergy Net energy: Produces unusable energy Heat loss: Significant StackFan Heat lost via radiant heating produces energy Heat lost inatmosphere produces energy Net energy: Produces unusable energy Heatloss: Significant Heat Balance Heal Produced 100% Heat used to producesteam  82% Heat loss through radiant convection  5% Heat loss dischargeto atmosphere  11% Heal loss from ash  2% Heat loss through combustor,<0.001% fire tube and boiler

To ensure that the heat loss from heating unnecessary air is minimized,the quantity of air at each point of the combustion process isstringently controlled. This air control benefits the combustion processin three ways.

The first way is by minimizing the heat loss in the combustion process.This minimizes the amount of carbon monoxide produced. Carbon monoxide,a priority pollutant, is produced directly proportional to thecombustion temperature. Therefore, by minimizing excess air we minimizethe quantity of carbon monoxide.

Second, nitrogen oxides, another priority pollutant, is produced bycombining the nitrogen in air with the oxygen in the air. Thiscombination of nitrogen and oxygen only occurs at high temperatures. Thehigher the temperature, the greater the quantity of nitrogen oxides thatis produced. While the combustor disclosed herein utilizes very hightemperatures, the formation of nitrogen oxides is minimized since thereis no excess oxygen to combine with the nitrogen. All of the oxygen isused in the combustion process.

Third, by minimizing the amount of air supplied to the combustionprocess, this also minimizes the amount of energy required to move theair to the combustor. Electrical energy costs are typically 20% lessthan electrical energy costs in similar combustion systems where the airis not stringently controlled.

Reflection of Infrared Energy

Heat reflection is another innovative feature of the combustor of thisinvention. Heat is a form of electromagnetic energy, similar to lightwhere the rays can be refracted or reflected. Radiation produced fromheat is of a longer wavelength than visible light and is called infraredradiation.

By reflecting a certain amount of the heat produced from the combustionprocess, we can supply additional heat to the gasification process. Thereflected heat will be of benefit in two ways:

The first way is that by the heat reflected to a point where the heatcan be used to assist the combustion process. This is generally wherethe fuel first enters the combustion process. At this point, the fuelmust be heated and the water removed. These processes require theaddition of energy which can be added from either the combustion of apart of the fuel or from the reflected energy. Using a part of the fuelto preheat the remaining fuel is inefficient, leaving less total heatavailable for the production of electricity. Using the reflected heatremoves or minimizes this inefficiency.

The second way reflected heat benefits the overall combustion process isthat the energy is transferred in a beneficial way and is not wasted byirradiating and heating the combustion chamber. Heat that is absorbed bythe combustion chamber is generally wasted since there is no directbenefit from this radiation. A small portion is used in the maintenanceof the necessary combustion temperature, but the majority of theradiative heat is wasted as low level heat radiated from the combustorexterior. The reflection of the heat back onto the fuel will benefit theoverall combustion efficiency.

Gasifier Firetube with Connection to Boiler 12

This is actually a part of the gasifier and is referred to as stage 2.It is the connecting tube to the boiler 13. Outside ambient air ispreheated between outer skins of the gasifier and injected at a ratecontrolled by the computer control system. When this oxygen rich airmeets the gas from the gasifier, ignition takes place in the firetubeand thus enters the boiler. The following parameters are taken in thefiretube for the computer control system's use:

Stage 2:

-   -   Air flow rate,    -   Oxygen concentration,    -   Carbon dioxide concentration,    -   Carbon monoxide concentration,    -   Firetube draft pressure,    -   Firetube temperature,    -   Boiler draft pressure,    -   Boiler temperature.        Boiler 13 (See FIG. 7B):

The boiler 13 (FIG. 4) uses the thermal energy to generate steam. Thefollowing parameters (see FIG. 7B) are taken in the boiler for thecomputer control system use for control of the feed rate and the steamoutput:

-   -   Air flow rate,    -   Steam pressure,    -   Steam temperature.

The boiler 4-10 (13) used in this embodiment of the invention is of theScotch Marine Type of boiler. Other types of boilers such as A-frame,H-frame may be used in other installations.

The boiler 4-10 absorbs the heat produced from the combustion of thefuel and transfers it to water, which is converted into steam. The steamis used to produce mechanical work such as electrical generation,heating, etc.

The Scotch Marine Boiler is a three-pass type of boiler. The first passis through a large diameter central tube 4-11. Approximately 40% of theusable heat is absorbed during the first pass. This heat is used toheat, the water and to convert part of the water into steam. The secondpass is in the reverse direction through a series of small tubes 4-12.An additional 40% of the usable heat is absorbed during the second pass.The remaining water is converted into steam by the heat absorbed duringthe second pass. The third pass is again through a large diameter tube4-14 where the remaining 20% of the heat is absorbed. This heat is usedto, in essence, superheat the steam and ensure that no liquid waterremains.

An economizer may be attached after the boiler to preheat the water andimprove efficiency. This is not shown in this embodiment.

Cyclone 14:

The cyclone 14 (FIG. 5) is the first mechanical device that removesparticulate. The design of the cyclone is such that when the air flowsthrough it from the boiler the largest particulate will drop from theairflow through the bottom of the cyclone to a storage container. Thefollowing parameters are measured (see FIG. 7B) for the computer controlsystem 17 in the cyclone:

-   -   Inlet temperature,    -   Inlet pressure,    -   Outlet temperature,    -   Outlet pressure.

The cyclone 5-10 and baghouse 5-11 operate as particulate collectiondevices. In the combustion process, as the fuel is combusted a smallpercentage of ash remains. Some of this ash is entrained within thecombustion air stream and is carried along with the exhaust gases,called fly-ash. The cyclone 5-10 and baghouse 5-11 remove the fly-ash sothat it is not emitted into the atmosphere.

The cycle acts through centripetal action. The gas spins around in thecyclone and separates the heavy particles from the gas based uponweight. The ash particles are collected at the bottom of the cyclone andremoved through a rotary air lock and a vacuum removal system (eductor).

The baghouse operates on a different principle. The gas passes through aseries of fiberglass bags 5-13 that have very small openings withinthem. The gas can pass through but the particles cannot and remainadsorbed to the exterior of the bag. At appropriate intervals,high-pressure air is introduced inside the bags. This air literallyforces the particles off of the bags and they fall to the bottom of thebaghouse. The particles then are collected through a rotary air lock andan eduction system similar to the cyclone.

Baghouse 15:

The baghouse 15 (FIG. 5 is the final removal equipment for particulate.The mesh size of the bags will be determined by the particulatedischarge requirements. The following parameters are taken in thebaghouse for the computer control system use:

-   -   Inlet temperature,    -   Inlet pressure,    -   Outlet temperature,    -   Outlet pressure.        Scrubber (FIG. 6):

The scrubber 6-10 is used to remove acid gases from the exhaust gasstream. A mixture of lime (calcium oxide, a strong caustic) and water issprayed 6-11 through the exhaust gas. This liquid chemically reacts withthe acid gases such as sulfur dioxide, hydrogen chloride, etc. to removethe acid gases. Plastic or ceramic open-frame balls are often used aspacking to increase the surface area of the contact surface to improvethe efficiency of the chemical reaction. After the liquid has reactedwith the gas, the gas stream passes through a series of impediments,called demisters 6-13 to remove all excess liquid. The gas then proceedsto the stack exhaust fan where the clean exhaust gas is vented to theatmosphere.

After the liquid has reacted with the acid gases, it is collected in aspent slurry collector and returned for treatment to a source by pump6-14 in the bottom of the scrubber where it is pumped to a separationchamber and the lime solution recycled.

The scrubber 6-10 (FIG. 6 ) cleans the bad gases from the air streambefore discharge to the atmosphere. It is usually a wet or dry limeinjection system depending on the discharge requirements. The followingparameters are taken for the computer control system that thendetermines the feed rate for the catalytic agent:

-   -   Particulate (opacity),    -   Sulfur dioxide,    -   Nitrogen oxides,    -   Carbon monoxide,    -   Volatile organic carbon,    -   Certain hydrocarbon species,    -   Hydrogen chloride,    -   Hydrogen fluoride,    -   Hydrogen sulfide.

Cleaned exhaust gases are fed by fan 6-17 to the exhaust stack.

Control System 17:

The computer control system (see flow charts in FIGS. 7A, 7B and 7C) isthe determining factor for the gasification system to operate properlyand to be in compliance with the regulatory requirements for airdischarge. It includes Programmable Logic Controllers (PLC'S) andvariable speed drives, diagrammatically illustrated in FIGS. 7A, 7B and7C, that operate the various motors, fans and drives that operate thegasifier system. The PLC'S are in turn controlled by signals from acomputer that is programmed to recognize all the variables listed plusother minor items and to react properly from the data base. The programis designed to make adjustments for different types of fuels (withdifferent BTU content) without changing equipment in the gasifiersystem.

Combustor Process Description

Thermodynamic Extraction of Chemical Potential Energy

The release of chemical potential energy is a two-stage process: Stage 1gasifies the carbon-based solid fuels and Stage 2 oxidizes the gasifiedfuels to produce heat.

Stage 1 is subdivided into two separate processes involved in thegasification of solid fuels. Stage 1 a uses thermal decomposition of thesolid fuel introduced into the combustor to break the fuel into gaseousfractions of lower molecular weight or elemental composition. Allabsorbed compounds in the fuel such as water and other solvents arereleased in this stage.

A polymerized hydrocarbon based fuel (both plastic and lignin/cellulosebase fuel) is decomposed into short chain aliphatic hydrocarbons,elemental carbon, carbon monoxide and hydrogen through the addition ofenergy as heat. Other elemental based polymers including sulfur andsilicon based compounds are similarly broken into appropriate monomersor elements using the same process. The ash produced from Stage 1 a islargely carbon with small amounts of metal oxides.

The heat required for the endothermic decomposition of the fuel isproduced from heat supplied from stage 1 b and from limited oxidation offuel in stage 1 a. The oxidation of the solid fuel is limited in thisstage by careful control of air added to the combustion process in Stage1 a. The amount of air injected into Stage 1 a is controlled by theamount of oxidation required to maintain the minimum necessarydecomposition temperature in this stage.

Stage 1 b utilizes an exothermic partial oxidation of the carbon in theash to produce carbon monoxide and heat. The remaining solid ashconsists entirely of metallic oxides. The reaction in Stage 1 b islimited to partial oxidation of the carbon by controlling the airinjected into Stage 1 b.

The heat of reaction of the carbon oxidation is used in Stage 1 a todecompose the fuel as previously described. Approximately 80% of theheat of reaction is utilized in this process with the remaining heatpassing to Stage 2.

Physical Process

Solid fuel is introduced to the combustor section 1 a (firebelt 20)where gasification and moisture removal occurs. A minimal amount of airis introduced to Section 1 a to maintain the minimum gasificationtemperature necessary for the specific type of fuel used. Radiativeenergy from Section 1 b, firebelt 21 also added to the energy requiredfor gasification.

The gases exiting this section consist of primarily carbon monoxide,hydrocarbons (short-chain and long-chain) and water vapor with minimalquantities of carbon dioxide and the balance of nitrogen. The ashproduced through the gasification process consists of carbon,long-chain, high-boiling-point hydrocarbons and metallic oxides.

Control of the gasification process is accomplished by modulation of thefuel feed rate, the quantity of air introduced through firebelts asmeasured by the oxygen concentration, the gasification temperature andthe speed of the firebelts. Air injected into the solid fuel isminimized to prevent quenching of the air/fuel reaction and to preventcomplete oxidation of carbon to carbon dioxide. The firebelt speed iscontrolled so that the solid fuel has been completely gasified at theend of the belt.

The carbon ash from firebelt 20 falls onto section 1 b (firebelt 21)where additional air, in decreasing quantities, is supplied to combustthe carbon to carbon monoxide as well as decomposition of the long-chainhydrocarbons to carbon monoxide and hydrogen. Section 1 b gases consistof carbon monoxide (10-15%), hydrogen (5-20%) and carbon dioxide (1-10%)with the balance of the gas being nitrogen. The ash remaining from thisprocess consists of metallic oxides with trace quantities ofcarbon-based compounds. Oxygen content is minimized in stage 1 b toprevent oxidation of the carbon to carbon dioxide. The firebelt speed iscontrolled to ensure complete oxidation of the ash just before the endof the belt.

Infrared radiation energy produced from stage 1 b is reflected off ofthe refractory walls onto section 1 a where it is used to heat andgasify the solid fuel. Control of the section 1 b process is performedby the firebelt 21 speed, stage 1 b temperature, control of the air tofuel ratio through firebelt 21 as measured by the oxygen concentrationand by the overall draft (negative pressure) of the combustor system.

Stage 2 combustion occurs within the firetube and within the boilerwhere the carbon monoxide and hydrogen gases are oxidized to carbondioxide and water by the addition of additional air. The firetube isused for mixing of the air and fuel gas with final combustion occurringwithin the boiler cavity.

The Stage 2 combustion process is controlled by the air/fuel gas ratio,boiler temperature, firetube temperature, carbon monoxide concentration,oxygen concentration, carbon dioxide concentration and by the overalldraft of the combustor system.

Pollution Control

The combustor of the invention is a remarkably simple combustion system,and it is very easy to minimize the air pollutants that are producedfrom the combustion of solid fuels. This is not the case with othercombustor systems currently on the market. With two exceptions, thecombustor disclosed herein is the only combustion system that does notrequire a gaseous fuel afterburner to remove excess pollutants. Theremaining two combustion systems are on the order of ten times asexpensive to accomplish the same degree of pollution control as thecombustor of this invention.

There are five different types of air pollutants that the EnvironmentalProtection Agency regulates in solid fuel combustion systems. Thecombustor disclosed herein has been specifically designed to minimizethe quantity of each five types of pollutants. Each of these fivecategories will be discussed individually.

Particulates

Particulates are released into the atmosphere from materials in the fuelwhich cannot be burned. Usually these particles are a chemical part ofthe fuel and when burned, recombine as small particles. Part of theseparticles agglomerate together in chunks which then collect in thebottom of the combustor and are removed as bottom ash. The remainingparticles are carried in the flue gas.

These particles could be released into the atmosphere unless they areremoved. In the combustor of this invention, the particles are removedby a device called a baghouse. The baghouse is a large chamber filledwith cloth bags that collect the dust as the gas passes through them.The dust is then removed and the cleaned gas can be released into theatmosphere.

The technology of baghouse design and construction is well advanced.There have been very few refinements in the baghouse particulate removalsystem since the mid-1970's.

Sulfur Dioxide

Sulfur oxides (NO_(x)) are produced by the nitrogen combining withoxygen in the presence of high temperatures. Generally, the higher thetemperature, the higher the quantity of nitrogen oxides produced.Because nitrogen oxides have been found to be a contributing factor inthe destruction of ozone in the atmosphere, the emission of thesecompounds are regulated and must be minimized.

For most solid fuel combustion sources, a system of reducing nitrogenoxides must be added to the combustor to lower the nitrogen oxideemissions to an acceptable level. This reduction system uses theinjection of ammonia gas into the combustion system and a catalyticconverter (similar to today's automobiles) to remove the nitrogen oxidesand the ammonia. This is an expensive process, both in capital costs forthe previous metal catalyst and for operating costs of ammoniainjection. Additionally, another pollutant, ammonia, a highly toxiccompound, has been introduced into the atmosphere which must bemonitored.

The present combustor uses a different method to reduce nitrogen oxides.By closely controlling the amount of air that is introduced into thecombustion process, the formation of nitrogen oxides is minimized.Nitrogen oxides cannot form if there is not oxygen to combine with thenitrogen. In the combustion process disclosed herein, only enough air isadded to the fuel to perform the necessary combustion of the fuel.Because there is no additional oxygen, there are very low quantities ofnitrogen oxides produced. Excess air is added only at the very end ofthe combustion process to ensure complete combustion of the fuel. Usingthis process, very low concentrations of nitrogen oxides are emittedinto the atmosphere.

Carbon Monoxide

Carbon Monoxide (CO) is the result of incomplete combustion. This is dueto either low combustion temperatures or insufficient combustion air. Inthe combustor of this invention, the combustion temperature exceeds3,000° F. for all solid fuels and 4,000° F. using tires. To ensure thatthe combustion process is complete and no carbon monoxide remains, asmall amount of excess air is added to the final stage of combustion.This results in low concentrations of carbon monoxide and lowconcentrations of nitrogen oxides, a feat unable to be accomplished byany other combustor without an afterburner or a catalytic converter.

Volatile Hydrocarbons

Volatile hydrocarbons or volatile organic carbons (VOC) are a class ofcompounds that are regulated by the Environmental Protection Agency.These compounds include a wide range of chemicals that can be emittedinto the atmosphere. Included in this list are compounds like dioxins,polychlorinated biphenyls (PCBs), polynuclear aromatics (PNAS) and otherhazardous air pollutants (HAPS).

These compounds are created as the result of incomplete combustion. Inthe combustor of this invention, the formation of these compounds arekept to an extremely low level, in many cases unmeasurable due to theextreme temperatures present in the combustion process. It is highlyunlikely that the Environmental Protection Agency or any otherenvironmental regulatory agency will pass in the foreseeable future,regulations governing the release of volatile hydrocarbons that thepresent combustor will be unable to meet.

Monitoring of Pollutants

Regulations in all states and in most countries require a facility todemonstrate that they are complying with the applicable air emissionstandards. To demonstrate compliance, a facility must usually install asystem that continuously monitors the quality of the gas being releasedinto the atmosphere. The system is called a Continuous EmissionMonitoring System (CEMS).

Some of the parameters that are monitored include:

-   -   Particulates (opacity),    -   Sulfur dioxide,    -   Nitrogen oxides,    -   Carbon monoxide,    -   Volatile organic carbon,    -   Certain hydrocarbon species,    -   Hydrogen chloride,    -   Hydrogen fluoride,    -   Hydrogen sulfide.

The monitoring system continuously records the rate at which thepollutants are released into the atmosphere. These records must besubmitted to the regulators to prove that the facility did indeed meetthe required standards.

Combustor Control Description (FIGS. 7A, 7B and 7C)

The combustion control process is a series of nested control loops thatprovide the necessary regulation of heat production.

The primary loop that controls the heat production is regulated by thequantity of fuel that is admitted to the combustor. The fuel feed musthave a wide range of quantities due to the variety of fuels used in thecombustor.

Within the primary loop are combustion control loops that regulate thecombustion process in Stages 1 a, 1 b and Stage 2. This is controlled bythe speed of the combustion belts and the quantity of air added to thefuel as it is combusted. The goal of these control loops is to have thefuel completely consumed while maintaining the required pollutioncontrol.

All of the components controlled in the combustion system containfeedback to inform the computer control system if a componentmalfunctions. Different component types use different types of feedback;for example, the air control dampers include a position sensor so thatthe damper position set by the controller is returned to the controller.If the position of the damper differs from the setpoint, the operator isinformed and if the error is beyond a certain limit, the combustor isshut down.

Where possible, the computer control system is designed so that minorcomponent malfunctions are either self-corrected or the programmingcompensates for the error. If minor errors are noted by the computercontrol system, the system operator and system maintenance personnel arenotified for repair or replacement. This gives the computer controlsystem a great deal of intelligence including, where possible,predictive failures.

In summary, the following parameters are used to control the combustionprocess:

Stage 1 a:

-   -   Solid fuel feed rate,    -   Air flow rate,    -   Oxygen concentration,    -   Firebelt 20 speed,    -   Gasification temperature,    -   Draft pressure.

Stage 1 b:

-   -   Air flow rate,    -   Oxygen concentration,    -   Firebelt 21 speed,    -   Oxidation temperature,    -   Draft pressure.

Stage 2:

-   -   Air flow rate,    -   Oxygen concentration,    -   Carbon dioxide concentration,    -   Carbon monoxide concentration,    -   Firetube draft pressure,    -   Firetube temperature,    -   Boiler draft pressure,    -   Boiler temperature.

While the invention has been described in relation to preferredembodiments of the invention, it will be appreciated that various otherembodiments, adaptations and modifications of the invention will bereadily apparent to those skilled in the art.

1. In a solid fuel fed combustor having an inlet feed for solid fuel anda combustion chamber including a first burner stage having a firsttraveling conveyor firebelt with means for introducing air along thelength of said first traveling conveyor firebelt from below solid fuelon said first traveling conveyor firebelt and a second burner stagehaving a second traveling conveyor firebelt and means for introducingair along the length of said second traveling conveyor firebelt frombelow solid fuel on said second traveling conveyor firebelt, a method ofminimizing production of carbon monoxide and nitrogen oxide pollutantscomprising the step for controlling the amount of air introduced in saidfirst and second burner stages so that air in progressively increasingamounts is supplied to solid fuel along the length of said firsttraveling conveyor firebelt and air in progressively decreasing amountsis supplied to solid fuel traveling along the length of said secondtraveling conveyor firebelt and also the step for controlling the speedof said firebelts such that endothermic decomposition of solid fueltakes place in said first burner stage and exothermic combustion anddecomposition of solid fuel takes place in said second burner stage. 2.The method defined in claim 1 further including the steps for mixinggaseous products from said second burner stage with a controlled amountof heated air from ambient in a firetube, for combusting and theresulting mixture, and for using the released thermal energy to convertwater to steam in a boiler coupled to said firetube.
 3. The methoddefined in claim 2 wherein gaseous products and particulates from saidboiler are filtered in a cyclone and in a baghouse to removeparticulates, and in a scrubber for cleaning noxious gases beforedischarge to the atmosphere.
 4. The method defined in claim 1 whereinradiative energy generated during said exothermic combustion anddecomposition of solid fuel in said second burner stage is reflectedupon solid fuel traveling on said first traveling conveyor firebelt insaid first burner stage.
 5. In a solid fuel fed combustor having aninlet feed for solid fuel and a combustion chamber including a firstburner stage having a first traveling conveyor firebelt with means forintroducing air along the length of said first traveling conveyorfirebelt from below solid fuel on said first traveling conveyor firebeltand a second burner stage having a second traveling conveyor firebeltand means for introducing air along the length of said second travelingconveyor firebelt from below solid fuel on said second travelingconveyor firebelt, a method of minimizing production of carbon monoxideand nitrogen oxide pollutants comprising the step for controlling theamount of air introduced in said first and second burner stages suchthat a metered amount of air in progressively increasing proportion isintroduced along the length of said first firebelt and a metered amountof air in decreasing proportion is introduced along the length of saidsecond firebelt and also the step for controlling the speed of saidfirebelts such that endothermic decomposition of solid fuel takes placein said first burner stage and exothermic combustion and decompositionof solid fuel takes place in said second burner stage, and radiativeenergy generated by said exothermic combustion and decomposition ofsolid fuel is reflected on said solid fuel under endothermicdecomposition in said first burner stage.
 6. The method defined in claim5 further including the steps for mixing gaseous products from saidsecond burner stage with a controlled amount of heated air from ambient,for combusting the resulting mixture in a firetube, and for using thereleased thermal energy to convert water to steam in a boiler coupled tosaid firetube.
 7. The method defined in claim 6 including the steps forremoving particulates from gases from said fire tube, and using acyclone and a baghouse for cleaning noxious gases from the gases fromsaid firetube before discharge to the atmosphere.